Hi
I believe I've found a bug in your addon. This bug exists also in the twin plugin. (and in many other XBMC addons...)
If the path to the dll file contains a non ASCII letter, the adding will fail due to dll file not found.
I googled a bit and found this to be a known issue.
You can look here:
http://forum.xbmc.org/showthread.php?tid=125492
When I added a user name to my system that has only ASCII letters I had no problems with the addon.